1. Field of the Invention
The present invention relates to a system for controlling the transmission ratio of a transmission, and more particularly to a system for controlling the transmission ratio of a mechanically operated infinitely or steplessly variable transmission for automotive vehicles.
2. Description of the Prior Art
The transmission ratio control of the infinitely variable transmission has heretofore been accomplished by means of a transmission ratio ("ratio" hereinafter) control actuator upon which hydraulic ratio control pressure from a flow rate control valve is applied. The ratio control pressure is produced by a flow rate control valve which is differentially amplified by a force exerted by the displacement of a cam operatively connected with a throttle valve and a counteracting output pressure of a hydraulic governor driven by an output shaft of a transmission. It is very difficult to design such control system as the ratio control actuator is directly driven by the differentially amplified pressure from the flow rate control valve. In other words, it is very difficult to obtain on the one hand a high governor pressure sufficient to drive a spool of the flow control valve, on the other hand to provide a long cam displacement, to precisely determine the spring strength of the flow control valve and to provide a sufficiently long stroke. In addition, a large force to depress an acceleration pedal is required by the long displacement of the cam. It is preferable to obtain a compensation control hydraulic pressure by pressing a pilot piston by means of an additional cam linked with a throttle valve if pressures for compensation such as differential compensation for throttle opening and compensation at starting are additionally applied to the ratio controlling pressure, i.e. actuator driving pressure. Still a greater force is required for depressing the acceleration pedal to provide compensation pressure in such case. Furthermore, the design of flow control valve becomes still more difficult if a well-balanced hydraulic pressure for the control of actuator would be generated by means of only one flow control valve into which a compensation control pressure is also input. Such drawbacks in the prior art make it also difficult to obtain a reliable ratio controlling system covering a wide transmission ratio range.
Particularly in a mechanically operated infinitely variable transmission, operation of the ratio control actuator requires a very large force due to its encountering a counteracting force exerted on a torque transmitting portion of the actuator. Such ratio control actuator is generally actuated and driven through hydraulic pressure, wherein it is desirable that this hydraulic pressure is kept stable without a pressure change over the whole range of the transmission ratio. Therefore, it is not desirable that the signal pressures be responsive to the throttle valve, the speed of which or the like are directly applied for actuating the ratio control actuator because such pressures are accompanied by low-high pressure changes.
Furthermore, there is much to be desired in the prior art to overcome a force toward undesired ratio changes produced by wheels when they suddenly fall into a recess during running and a reacting force is transmitted to the ratio control actuator via a transmitting mechanism.
As general requirements for the ratio control system for the infinitely variable transmission the following requirements should be satisfied for obtaining a highest output torque in the transmission output shaft at high load time such as starting, rapid acceleration, upgrade-rising and the like: to established or shift a region in the ratio when necessary where the ratio is fixed at a minimum, or when necessary to establish or shift a region where the ratio is fixed at a maximum. In this case, an infinitely variable region of the transmission ratio is to be established and shifted between both regions of the fixed low ratio region and fixed high ratio region while an infinite or stepless variable change is matched with an engine revolution number (r.p.m.) N.sub.E at each stage under a given vehicle speed.
Such requirements are essential for safe and effective running of an automotive vehicle, with which the infinitely variable transmission system should be provided for enabling extension of the fixed low ratio region towards a high speed or down shift at a middle or high speed under any circumstances requiring high output torque (high load) such as at starting, rapid or high acceleration, upgrade-ascending, or engine braking (at high speed running or downgrade-descending).